The present invention relates to a rotary anode type X-ray tube and an X-ray tube apparatus provided with a rotary anode type X-ray tube.
A rotary anode type X-ray tube comprises a disk-like anode target, a cathode structure for irradiating the target with an electron beam, a rotary structure for rotatably supporting the anode target, and a stationary shaft or structure for supporting the rotary structure with a bearing arranged therebetween, which are arranged within a vacuum envelope. A stator coil for generating a rotating magnetic field for rotating the rotator is arranged outside the vacuum envelope.
In general, the rotary anode type X-ray tube and the stator coil of the construction described above are housed in a vessel for housing an X-ray tube constructed such that an insulating medium is loaded and circulated therein. The structure of the particular construction is mounted as an X-ray tube apparatus in an x-ray tube system such as a CT scanner so as to be used. In the x-ray tube apparatus, the insulating medium that is allowed to flow through the clearance or gap between the rotary anode type X-ray tube and the housing vessel serves to ensure an electrical insulation among the members providing a large potential difference during the operation and also serves to cool the rotary anode type X-ray tube.
In the apparatus of the construction described above, the stator coil arranged outside the vacuum envelope generates a rotating magnetic field, and the anode target is rotated by the rotating magnetic field. Under this state, electron beams generated from the cathode are allowed to strike against the anode target, with the result that an X-ray is generated from the anode target.
In the rotary anode type X-ray tube, a bearing is arranged between the rotary structure and the stationary shaft. The bearing includes a roller bearing such as a ball bearing and a dynamic pressure slide bearing in which a spiral groove is formed on at least one of bearing surfaces faced to each other with a gap, and the bearing gap and the spiral groove is filled with a liquid metal lubricant such as gallium (Ga) or a gallium-indium-tin (Ga--In--Sn) alloy.
A rotary anode type X-ray tube using a dynamic pressure slide bearing is disclosed in, for example, Japanese Patent Publication (Kokoku) No. 3-77617, Japanese Patent Disclosure (Kokai) No. 3-182037, Japanese Patent Disclosure No. 5-144396, Japanese Patent Disclosure No. 8-241686 and U.S. Pat. No. 5,838,763.
In the dynamic slide bearing used in the rotary anode type X-ray tube, the small clearance or gap, e.g., a clearance of about 20 .mu.m, is retained between the bearing surfaces; and a liquid metal lubricant is loaded in the spiral groove and the clearance of the bearing. In this case, unless the liquid metal lubricant permeates uniformly over the entire region of the clearance of the bearing, it is impossible to obtain a sufficient dynamic pressure, resulting in failure to maintain a stable bearing operation. In the extreme case, the bearing surfaces bite each other to make the rotation impossible or to bring about breakage.
The dynamic slide bearing that was put to practical use in the past is of a cantilever structure. Therefore, the stress applied to an edge portion of the bearing stationary shaft fixed to the x-ray tube housing vessel is increased with increase in the weight of the anode target, giving rise to a problem in the mechanical stability. Also, if an unbalance in pressure is generated by the centrifugal force received by the liquid metal lubricant in the bearing portion, the liquid metal lubricant tends to leak from the bearing, with the result that the disk-like anode target is considered to fail to rotate smoothly.
In order to overcome the problem described above, a dynamic slide bearing of a support structure for supporting the stationary shaft at both side is disclosed in, for example, U.S. Pat. No. 5,838,763. In the apparatus disclosed in this prior art, both sides of the stationary shaft of the bearing extend along the axis of rotation so as to be coupled with the vacuum envelope. Also, the stationary shaft is made hollow to permit a cooling medium to flow through the central bore.
In the rotary anode type X-ray tube of the construction described above, it is necessary arrange the cathode structure sufficiently apart from the anode target or the extending portion of the bearing stationary shaft because a large potential difference is provided therebetween, giving rise to the inconvenience that it is unavoidable to enlarge undesirably the length in the axial direction and the diameter in the radial direction of the X-ray tube.